Kharankui are giant spiders created by the Valg Queen Maeve. They make spider silk that is very rare and expensive. There are two types of Kharankui; the ones in the Ruhnn Mountains called Stygian Spider and the ones in the Southern Continent, the Kharankui. The Stygian spiders in Erilea make spider silk, whereas the Kharankui in the Southern Continent guard the Wyrdgate for Maeve and do not bother with making spider silk and trading.
In Throne of Glass, we come across Falkan Ennar who traded twenty years of his life for spider silk from Kharankui in the Stygian Mountains. In The Assassin and the Desert he met with Aelin who still went under the name of Celaena and told her the story. Falkan explained that the Kharankui told him that the only way for him to get his twenty years back was to find someone who would kill the spider.
The second person who did not trade, but stole spider silk from Kharankui was Manon Blackbeak. She stole the spider silk from the spider, and the spider promised to hunt her down if she did not offer her something in return. When Manon asked what she would want, the spider answered that Manon could give the spider her gold eyes, the way her voice sounds or her moon-white hair. Manon refused, but later reunited with the same Kharankui in Kingdom of Ash when she learned that the spider could shapeshift. Dorian decided to leave the spider alive and to teach him how to shapeshift, but he later killed her. It was also revealed that the spider Dorian killed was actually the same spider that took twenty years of Falkan's life.
At the same time across the world, Falkan got his youth back. After Aelin reunited with Chaol Westfall, Borte rushed in to tell Sartaq and Nesryn Faliq that Falkan finally got his years back, and that he looked like he was twenty years old again.